1. Field of the Invention
The present invention relates to an overvoltage protection circuit and an electronic device equipped with a built-in overvoltage protection circuit.
2. Description of the Related Art
In recent years and continuing, electronic devices such as mobile phones and digital cameras equipped with built-in secondary batteries are becoming popular. Such an electronic device is configured to receive power supplied from an AC adapter. The power from the AC adapter is also used to charge the built-in secondary battery. With the growing popularity of these electronic devices, AC adapters with different output voltages are becoming readily available. However, there are only a few variations in the shapes of the output terminals of the AC adapters. Accordingly, an electronic device may be incorrectly connected to an AC adapter that outputs an inappropriate voltage for that electronic device. For example, if an electronic device that operates at low voltage is connected to an AC adapter that outputs high voltage exceeding the withstand voltage of the semiconductor circuit provided in the electronic device, the semiconductor circuit in the electronic device will break down.
Japanese Laid-Open Patent Application No. 2002-218645 (Patent Document 1) discloses an overvoltage protection circuit that has been invented to avoid such a situation. FIG. 1 is a circuit diagram for describing the overvoltage protection circuit disclosed in Patent Document 1.
An overvoltage protection circuit 10 receives power supplied from an AC adapter 11. Output of the overvoltage protection circuit 10 is supplied to a device main unit 12.
The overvoltage protection circuit 10 includes resistances R11, R12 for detecting the output voltage of the AC adapter 11, a resistance R13 and a Zener diode D11 for generating a reference voltage, a comparator 14 for comparing the output voltage of the AC adapter 11 and the generated reference voltage, and a switch M11 that is caused to turn ON/OFF according to the output of the comparator 14. A resistance R14 is a bias resistance connected to the output of the comparator 14 and a diode D12 is a parasitic diode of a PMOS transistor comprising the switch M11.
When an appropriate voltage is output from the AC adapter 11 to the device main unit 12, the voltage of an input 1 of the comparator 14 is lower than the voltage of an input 2 of the comparator 14, and output of the comparator 14 becomes low (hereinafter, “L level”). Accordingly, the switch M11 is turned on and the output voltage of the AC adapter 11 is supplied to the device main unit 12.
If the AC adapter 11 outputs a high voltage that is inappropriate for the device main unit 12, the voltage of the input 1 of the comparator 14 becomes higher than or equal to the input 2, and the output of the comparator 14 becomes high (hereinafter, “H level”). Accordingly, the switch M11 is turned off and the output voltage of the AC adapter 11 is prevented from being supplied to the device main unit 12.
FIG. 2 is a circuit diagram for describing an overvoltage protection circuit disclosed in Japanese Laid-Open Patent Application No. 2002-313949 (Patent Document 2). This overvoltage protection circuit is integrated on part of a semiconductor substrate.
An integrated circuit 20 is fabricated by mounting an overvoltage protection circuit 21 on the same semiconductor substrate as a CMOS integrated circuit 22. Terminals 23, 24 are external terminals of the integrated circuit 20, which terminals are connected to a power source. Terminals 25, 26 are internal terminals of the integrated circuit 20 and act as power source terminals for the CMOS integrated circuit 22.
The overvoltage protection circuit 21 includes resistances R21, R22 for detecting the voltage of the power source, a PMOS transistor M21 and a resistance R23 serving as an inverter, and a switch M22.
When the voltage applied between the terminal 23 and the terminal 24 is an appropriate voltage, the voltage drop across the resistance R21 is less than a threshold voltage of the PMOS transistor M21, and therefore, the PMOS transistor M21 is turned off. This causes a connecting node between the drain of the PMOS transistor M21 and the resistance R23 to have a voltage of 0 V, so that the switch M22 configured with a PMOS transistor is turned on. Thus, the voltage applied on the terminal 23 is supplied to the terminal 25, so that this voltage is supplied to the CMOS integrated circuit 22.
When the voltage applied between the terminal 23 and the terminal 24 is an inappropriate voltage, the voltage drop across the resistance R21 increases. When this voltage drop exceeds the threshold voltage of the PMOS transistor M21, the PMOS transistor M21 is turned on. This causes the connecting node between the drain of the PMOS transistor M21 and the resistance R23 to have a voltage that is substantially the same potential as the voltage applied on the terminal 23. Accordingly, the switch M22 is turned off, so that the voltage applied to the terminal 23 is prevented from being supplied to the terminal 25. Japanese Laid-Open Patent Application No. 2003-303890 (Patent Document 3) discloses an overvoltage protection circuit in which the resistance R22 shown in FIG. 2 is replaced by a Zener diode.
Patent Document 1: Japanese Laid-Open Patent Application No. 2002-218645
Patent Document 2: Japanese Laid-Open Patent Application No. 2002-313949
Patent Document 3: Japanese Laid-Open Patent Application No. 2003-303890
However, in the above-described conventional overvoltage protection circuits, the switching element comprising the switch needs to be a transistor with high breakdown strength. Furthermore, all of the electric currents to be consumed by the device main unit 12 or the CMOS integrated circuit 22 need to flow through the switching element.
Such a transistor having high breakdown strength and superior current driving capabilities requires a large element area. For example, a transistor having a maximum rating of 15 V as the breakdown strength and a current driving capability of 850 mA requires an element area that is ten times as large as that of a transistor having a maximum rating of 7 V as the breakdown strength and the same current driving capability. Thus, in order to fabricate an IC with the conventional overvoltage protection circuit, an extremely large chip area is required, which leads to high cost.
Furthermore, when an inappropriate AC adaptor is connected to an electronic device, the power is prevented from being supplied to the main unit of the electronic device. Accordingly, the electronic device completely stops operating, which may cause a misperception that a failure has occurred in the electronic device.